This invention relates to air separation and in particular to an integrated method of separating air and generating power, and integrated plant for performing such a method.
A gas turbine comprises an air compressor, a combustion chamber and an expander. In operation, air is compressed in the compressor and is used to support combustion of a fuel gas in the combustion chamber. The resulting gaseous combustion products are then expanded in the expander or turbine with the performance of external work. This work may be the generation of electricity. Thus, the gas turbine may form part of a power station with the rotors of the compressor and expander and an alternator all mounted on the same shaft.
Commercial processes for the separation of air first require its compression. It is known to bleed compressed air from the air compressor of a gas turbine to feed an air separation plant. In a conventional air separation process, air is compressed, is purified by the removal of components such as water vapor and carbon dioxide that are less volatile than its main components, cooled to a temperature suitable for its separation by rectification, and then rectified in a so-called double rectification column having a higher pressure and a lower pressure stage. The oxygen product is typically withdrawn from the lower pressure stage as a vapor and warmed to ambient temperature by heat exchange with the incoming air. The lower pressure stage is conventionally operated at a pressure a little above atmospheric pressure so that the oxygen product is obtained at about atmospheric pressure. In some schemes, oxygen product from the air separation plant is used in the generation of the fuel gas that is burned in the combustion chamber of the gas turbine. Such processes typically require the oxygen to be produced at elevated pressure. Although the necessary pressure can be created by compressing the oxygen, U.S. Pat. No. 4,224,045 discloses that there are advantages in terms of the operating efficiency of the air separation process to operate the lower stage of the double rectification column at pressures well above atmospheric pressure. Further, the compressor of a gas turbine typically has an outlet pressure in the order of 10 to 20 atmospheres which is in excess of that required by the air separation process when the oxygen is taken from the lower pressure stage of the double rectification column at a pressure a little above atmospheric. Accordingly, it is typically desirable to operate the higher pressure stage of the double rectification column at substantially the same pressure as the outlet pressure of the compressor of the gas turbine.
Not only is oxygen then produced at a pressure well above atmospheric pressure, so is a nitrogen product. There are a number of proposals in the art including U.S. Pat. No. 4,224,045 for taking a stream of this relatively high pressure nitrogen product, warming it to about ambient temperature by heat exchange with the incoming air to about ambient pressure, further compressing the stream, further raising the temperature of the stream in a second stage of heat exchange with the incoming air so as to remove heat of compression from such air and then introducing the nitrogen into the combustion chamber or expander of the gas turbine. Accordingly, the nitrogen helps to power the gas turbine and therefore compensates for the loss of the air taken for separation from the air compressor of the gas turbine. Other examples of such processes are given in U.S. Pat. No. 4,557,735 and U.S. Pat. No. 4,806,136. One practical example of the above-described method is in the gasification of coal and is discussed in a paper entitled "Air Separation Integration for GCC Plants", by Olson, Jr, Anand and Jahnke, Tenth EPRI Conference on Coal Gasification Power Plants, 16 to 18 October 1991, San Francisco. In the integrated process described in this paper, nitrogen from the air separation plant is saturated with water vapor before being introduced into the turbine. We believe one purpose of this moisturizing is to provide additional returning mass to the turbine so as better to compensate for the air from the compressor of the turbine that by-passes the combustion chamber and flows into the air separation plant.
There is increasing interest in using pure oxygen or oxygen-enriched air together with coal in processes which form iron by the reduction of iron ore. It has for example been proposed to inject coal together with oxygen or oxygen-enriched air into the tuyeres of a conventional blast furnace thereby reducing the demand of these processes for coke and hence reducing the need for the operation of coke ovens which are viewed as providing environmentally harmful waste products. See for example a paper entitled "Oxy-coke Injection at Cleveland Ironworks". D A Campell et al, 2nd European Ironmaking Congress, Glasgow, September 1991, pp 233-246. Alternative processes using both oxygen and coal, such as the COREX process, eliminate the need for coke altogether. Such processes produce a fuel gas as a by-product, although the fuel gas does not have the high calorific value of one produced by the direct gasification of coal. Indeed, current proposals for enhancing the operation of a blast furnace by use of oxygen and coal typically produce a fuel gas by-product having a calorific value of less than 5 MJ/m.sup.3. Nonetheless, sufficient fuel gas is generated to make worthwhile its combustion for the generation of power. Thus, the fuel gas can be burned in a combustion chamber of a gas turbine and air taken from the compressor of the gas turbine for separation to form an elevated pressure oxygen product that is introduced into the blast furnace.
There is however a problem in introducing nitrogen into a gas turbine that employs a low calorific value fuel gas in its combustion chamber. The turbine has only a limited capacity for the return of pre-heated nitrogen and so only a part of the heat in the bleed air stream can be used for heating nitrogen. In addition, current gas turbine generally have fuel gas handling systems not able to handle gas at a temperature above 300.degree. C. Accordingly, it is desirable to keep the temperature of any nitrogen stream introduced into the combustion chamber of the gas turbine at or below 300.degree. C., and therefore a further limit is placed on the transfer of heat to such a nitrogen stream.
There is therefore a need for a method and apparatus which enables integrated air separation-gas turbine technology to be used when the fuel gas supplied to the gas turbine is of low calorific value, its source being for example a blast furnace. The invention aims at providing a method and plant that meet this need.